The present invention relates to a radiographic imaging system whereby a plurality of projection images (radiographic images) are acquired by tomosynthesis imaging to reconstruct a tomographic image of a subject in a given cross section from the acquired projection images.
In tomosynthesis imaging, a radiation source is moved in one direction, for example, as the subject is irradiated from different angles, and a radiation detector detects the radiation having passed through the subject to successively acquire a series of projection images taken from different angles. Subsequently, the acquired projection images are shifted and the pixels are superimposed so as to be in alignment for a structure of interest, thereby reconstructing a tomographic image in a given cross section of the subject.
Tomosynthesis imaging uses the fact that structures overlie over each other differently among projection images depending on the imaging angle and shifts and adds up the projection images appropriately as described above to obtain an image (tomographic image) where a structure of interest in a desired tomographic plane is enhanced.
However, in tomosynthesis, the range of the imaging angle is so limited as to reduce the separation ability in the depth direction. This may cause a problem of an artifact produced in a reconstructed tomographic image by an effect of another structure than the structure of interest.
JP 5-49631 A, prior art literature that may be related to the present invention, describes dividing an image into small regions each having 4×6 pixels and calculating the similarity between two images in each of the small regions in order to measure the amount of movement from a subject represented in two images. JP 7-37074 A describes using the normalized cross-correlation method to determine a sub-region in search area having a high similarity to image data in the template.
To enhance diagnosis performance of a reconstructed tomographic image, use of image processing has been proposed including frequency enhancement for enhancing a high frequency component and frequency suppression for suppressing a low frequency component as exemplified by a flow image contained in a tomographic image (JP 3816151 B).
Frequency enhancement is a processing whereby an unsharp mask image signal of an unsharp mask image (average image of a tomographic image) is subtracted from the image signal of a tomographic image to produce an image signal of a frequency image containing a high frequency component, and the frequency image signal is multiplied by an enhancement coefficient to enhance the high frequency component and added to the image signal of the tomographic image (JP 55-163472 A and JP 55-87953 A). Thus, the high frequency component in a tomographic image is enhanced.
The flow image is an obstruction shadow occurring in a tomographic image as an image of a portion where the dose of transmitted radiation existing elsewhere than in a focused cross section to be imaged greatly changed is produced along the direction in which the radiation source moves (JP 3-276265 A). The frequency suppression processing removes a low frequency component corresponding to, for example, a flow image in a reconstructed tomographic image, to produce an image from which the flow image, for example, has been removed.
To enable image processing such as frequency enhancement and frequency suppression, there is known a filtered back-projection method for performing filter correction as a tomographic image is reconstructed. For the filter correction, general use is made of, for example, a lamp filter having filter characteristics to perform linear conversion as shown in FIG. 17A or a filter obtained by multiplying that lamp filter by a window function such as Hanning window as shown in FIG. 17B.
The filter correction is typically implemented with a linear filter having high frequency enhancing characteristics and hence may develop an overshoot or an undershoot in the periphery of, for example, a metal, or an enhanced noise component because of an excessively enhanced high-contrast signal, possibly leading to occurrence of an artifact in the tomographic image. As an alternative method of reconstruction in tomosynthesis is also known a successive approximation method exemplified by algebraic reconstruction method but such method is prone to increased computation time.